Bulk Email

I am using Google Apps Engine (GAE) for a small project. I need to send
a bulk announcement email (1000 users) to everyone that has registered
on the site. The announcement has certain customizations for each user.
I have found that the mailer built into GAE doesn't make it through most
spam filters. Anyone have any suggestions on how I might be able to get
around this issue?
Since GAE cannot open sockets except to fetch web pages on port 80 and
443, I started using postmarkapp.com to send confirmation emails.
However, they have restrictions against bulk email and only allow
transactional email. I tried sending a bulk email, but they detected it
pretty quickly and shut it down.
Lane